Heretofore, amorphous thermoplastic pipe such as chlorinated polyvinyl chloride ("CPVC") and polyvinyl chloride ("PVC") have been discharged from the extruding apparatus at a size approximating the size of the final product. Since the product undergoes melt swell as the result of viscoelastic recovery after being discharged from the apparatus, the extrudate is pulled from the extruding apparatus at a rate higher than the extrusion rate in order to draw the extrudate to a size more closely corresponding to the final product size. The extrudate is subsequently sized to the final size.
U.S. Pat. No. 4,613,532 to Michel et al teaches that polyamide pipe extrudates may be sized through a die having a diameter in excess of the final product diameter, cooled, and resized adiabatically through a disk having a diameter less than the final diameter.
U.S. Pat. No. 4,152,380 to Graves et al teaches that the inside diameter of extruded plastic pipes and the wall thickness is controlled by the takeaway speed at which the molten plastic pipe is removed from the sizing sleeve.
However, the prior art does not address the problem of internal stresses introduced during extrusion and post extrusion processing.